Higher-order chromatin structures are of profound importance in gene regulation and epigenetic inheritance (Wu and Grunstein (2000) Trends Biochem. Sci. 25:619-623). Post-translational modifications of core histones critically influence the establishment and maintenance of higher-order chromatin structures. The unstructured tails of certain core histones are extensively modified by acetylation, methylation, phosphorylation, ribosylation and ubiquitination. A “histone code” hypothesis, linking histone modifications to chromatin structures, has been the focus of intensive recent studies (Strahl and Allis (2000) Mol. Cell. Biol. 22:1298-1306; Turner (2000) Bioessays 22:836-845). Histone methylation has emerged as a major form of histone modification. (Strahl and Allis (2000) Mol. Cell. Biol. 22: 1298-1306; Zhang and Reinberg (2001) Genes Dev. 15:2343-2360). In particular, a large family of SET domain-containing histone methyltransferases (HMTases) has been identified (Lachner and Jenuwein (2002) Curr. Opin. Cell Biol. 14:286-298). SET domain proteins have been shown to methylate various N-terminal lysine residues of histone H3 and H4. Histone lysine methylation has been associated with diverse biological processes ranging from transcriptional regulation to the faithful transmission of chromosomes during the cell division (Grewal and Elgin (2002) Curr. Opin. Genet. Dev. 12:178-187).
Further, lysine methylation catalyzed by SET domain containing proteins has been linked to cancer (Schneider, et al. (2002) Trends Biochem. Sci. 27:396-402). For example, the H3-K4 methyltransferase MLL is frequently translocated in leukemia (Ayton and Cleary (2001) Oncogene 20:5695-5707; Milne, et al. (2002) Mol. Cell 10:1107-1117; Nakamura, et al. (2002) Mol. Cell 10:1119-1128) and the H3-K27 methyltransferase EZH2 is overexpressed in a number of tumors and its expression level correlates with the invasiveness of these tumors (Bracken, et al. (2003) EMBO J. 22:5323-5335; Kleer, et al. (2003) Proc. Natl. Acad. Sci. USA 100:11606-11611; Varambally, et al. (2002) Nature 419:624-9).
Chromosomal translocation is one of the major causes of human cancer, particularly in acute leukemias. The most common chromosome rearrangement found in leukemia patients involves the mixed lineage leukemia gene MLL (also called ALL or HRX) located at 11q23 (Ayton and Cleary (2001) Oncogene 20:5695-5707). MLL is the human homologue of Drosophila Trithorax (Trx), a protein involved in maintaining the “on state” of the homeotic box (Hox) gene expression during embryonic development. MLL contains a number of functional motifs including the N-terminal AT hook DNA binding motif and the C-terminal SET domain required for its H3-lysine 4 methyltransferase activity (Milne, et al. (2002) Mol. Cell 10:1107-1117; Nakamura, et al. (2002) Mol. Cell 10:1119-1128). As a result of chromosome translocation, MLL N-termini become fused in-frame with one of more than 30 partner proteins (Ayton and Cleary (2001) Oncogene 20:5695-5707). Regardless of whether the fusion partner is normally localized to the nucleus or cytoplasm, the chimeras are always nuclear (Dimartino and Cleary (1999) Br. J. Haematol. 106:614-626). Given that the DNA binding domain of MLL is still retained in the fusion proteins, the MLL target genes will be differentially regulated as a result of loss of the MLL SET domain and gain of fusion partner function in the chimeras. HOXA9 has emerged as one of the most relevant MLL target genes in human acute myeloid leukemia (AML) as it is always up-regulated in AML (Golub, et al. (1999) Science 286:531-537). Indeed, the leukemogenic potential of Hoxa9 was directly demonstrated by the development of AML in mice receiving transplantation of bone marrow cells overexpressing Hoxa9 (Kroon, et al. (1998) EMBO J. 17:3714-3725). Both Hoxa7 and Hoxa9 have been shown to be required for MLL fusion proteins to transform myeloid progenitor cells (Ayton and Cleary (2003) Genes Dev. 17:2298-2307). However, the mechanism by which different MLL fusion proteins up-regulate HOXA9 and how higher levels of HOXA9 leads to leukemia remains to be elucidated.
Dot1 is an evolutionarily conserved protein that was originally identified in S. cerevisiae as a disruptor of telomeric silencing (Singer, et al. (1998) Genetics 150:613-632). It also functions at the pachytene checkpoint during the meiotic cell cycle (San-Segundo and Roeder (2000) Mol. Biol. Cell. 11:3601-3615). Sequence analysis of yeast Dot1 revealed that it possesses certain characteristic SAM binding motifs, similar to the ones in protein arginine methyltransferases (Dlakic (2001) Trends Biochem. Sci. 26:405-407).